The present invention relates to a measurement apparatus and method for measuring characteristics of light emitting devices, especially of laser diodes used in an optical CATV equipment. In particular, the invention relates to an apparatus and method for automatically measuring a carrier-to-noise ratio, an optical modulation index, and a relative intensity noise of the laser diode.
Nowadays, the needs to evaluate characteristics of laser diodes used in an optical communication system, especially in an optical CATV (cable television) equipment have increased. The needs include to evaluate the laser diodes precisely, easily, and quickly, in conformity with each application. The reasons for these need include that, in an optical CATV equipment and in an optical fiber transmission media, multiplexed communication method tends to be used by incorporating optically modulated multichannel scheme, and thus, it becomes more important to guarantee the quality of audio and video data for all of the channels to subscribers.
Consequently, it is important for both manufacturers of the laser diodes and providers of the CATV equipment using the laser diodes, to precisely evaluate the characteristics of the laser diodes. But in reality, parameters are measured one-by-one separately by two or more measurement apparatus and the characteristics of the laser diode are obtained by calculating these measured parameters. Although some CATV systems have 40 channels or more in the frequency range of 50 to 500 MHz, at present, there is no single measurement apparatus and method that can automatically measure characteristic of these channels precisely, easily, within a short period of time.
FIG. 6 is a conceptual diagram of such an optical CATV multiplexed communication system. In FIG. 6, a plurality of channels, for example, TV channels, are mixed in a mixer whose output is connected to a laser diode. The light signal of the laser diode is modulated by TV signals and transmitted through a glass fiber to a receiver.
As technologies in such communication systems advance, the optical CATV multiplexed communication system incorporates more communication channels. Thus, it is necessary to improve test throughput for evaluating laser diodes for use in the CATV systems. One of the important laser diode characteristics is a relative intensity noise (RIN) in which a noise component of light emitted by a laser diode is evaluated with respect to an average light power of the laser diode.
In the prior art, the RIN is measured by the process as follows. First, the light signal from a laser diode is detected by a photo detector and the noise component in the light signal is measured by a measurement apparatus. Next, the average power of the light signal is measured by another measurement apparatus. Finally, the RIN is obtained by calculating these measured data and compensating the difference in the absolute value between the two measurement apparatus.
FIG. 12 is a block diagram showing an example of conventional arrangement for measuring the RIN. In FIG. 12, the light signal from a laser diode (not shown) is converted to an electric signal by a photo detector 201. The noise component is passed through a bias tee 202, amplified by an amplifier 204 with gain G, and measured by a spectrum analyzer 205. On the other hand, the average power of the light signal is measured directly as an electric current by an ammeter 203. The relative intensity noise (RIN) is expressed by the equation: EQU RIN={(P.sub.total -P.sub.th)/G-P.sub.sh }/(Ip.sup.2 RB) (1)
where P.sub.total is a total noise level, P.sub.th is a thermal noise level in the overall measurement apparatus, P.sub.sh is a shot noise level induced by the photo detector, Ip is a direct current (DC) of the photo detector, R is a load impedance of the photo detector 201 or an input impedance of the amplifier 204, and B is a measuring bandwidth.
P.sub.total and P.sub.th pass through the amplifier 204 and are measured by the spectrum analyzer 205. On the contrary, Ip is measured by the ammeter 203 without passing through the amplifier 204 because of a capacitor in the bias tee 202. In this situation, because the noise components and the average power of the light signal are measured separately through separate measurement paths, the measured data must be precisely compensated or calibrated so that the absolute values in the two measurements have accurate co-relationship one another.
As noted above, the prior art measurement apparatus has problems in which the correction or compensation in the measured values is inevitable to obtain the absolute values in the measuring procedure of RIN, which makes the RIN measurement complex and tedious. Moreover the resulted data tends to include errors because an error in each measuring procedure will be accumulated.